The present invention relates to pneumatic tire construction characteristics particularly applicable to radial ply tires. More particularly the present invention relates to a tire construction configuration which substantially reduces certain forces created by a tire in a straight ahead rolling condition which are particularly critical with respect to a radial ply tire configuration. More specifically, the invention relates to construction parameters for a radial ply tire which reduce the lateral forces on a radial tire known as ply steer to an insignificant magnitude without deleterious influence on other tire characteristics.
Ply steer and conicity are tire forces which are generated in a direction transverse to or laterally of the direction of rotation of a tire under load at the interface between the tire and road surface. Although the tire industry has been aware of the existence of these forces for many years, they have not been the subject of any significant extent of attention in terms of research or development inquiry for the reason that they assumed a relatively insignificant posture with respect to other considerations in the context of bias and bias-belted tires which have domestically constituted essentially the industry standards until very recently. Growing concern in an understanding of the nature and possible control of these forces has been substantially accelerated by the increased usage of radial tires since the magnitude and standard deviation in a plurality of tires in respect of radial tires is substantially greater than in the case of either bias or bias-belted tires. The relatively primitive status of the investigation of both ply steer and conicity forces is evidenced by the fact that there are no industry standards with respect to the interpretation, measurements or effects of these forces and that references thereto in the literature to date are limited virtually exclusively to speculation on the part of individuals carrying out research in regard to other tire and tire-vehicle interaction factors.
Concern is evidenced in the industry with respect to conicity; however, this lateral force component can normally be adequately managed as a matter of quality control. Since conicity in radical tires is believed to be primarily a function of belt centering, suitable construction standards can effect sufficiently accurate placement of the belts relative to the circumferential center line of a tire such that conicity can be maintained within acceptable limits. It is also significant that the conicity force developed by a tire does not reverse with a reversal of the direction of rotation thereof such that compensation for at least moderate irregularities can be effected by selective placement of tires on an appropriate side of car depending upon the conicity and total lateral force characteristics of that tire, the tire on the opposite end of the same axle, and the tires on the other axle of a vehicle.
In contrast, the ply steer force generated by a tire in rotating motion reverses upon reversal of the direction of rotation such that shifting of the tire to another position on a vehicle, if feasible, would not in and of itself produce a tendency to eliminate an undesirably high lateral ply steer force. Since ply steer forces in radial tires can readily achieve a magnitude of 60 to 70 pounds in what might be considered conventional radial tires of current vintage, a significant influence on the operating characteristics of a vehicle may be caused thereby. Primarily, ply steer manifests itself in terms of vehicle drift or dog tracking which is a condition wherein a vehicle operates at a constant yaw or sideslip angle to the direction of motion, thereby producing the observable result that the rear tires do not track in the path of the front tires. This may cause continual lateral scrubbing of the rear tires which is known to greatly accelerate wear.
Investigations which have been made in the industry have indicated that solution to the problem created by ply steer forces cannot be solved by selective loading, inflation, or control of rim width. Available information indicates that ply steer forces although variable to an extent with changing values of these functions remains quantitatively significant throughout a normal load range, over normal operating inflations and throughout a practical range of rim widths. Since these factors have been demonstrated to be insignificant in regard to the control of ply steer forces, speculation and theorization have largely focused on tire construction parameters which have been known to affect ply steer forces. For example, it has been theorized in the past that ply steer is primarily influenced by the tread, the outer stabilizer ply, or a combination of undefined influences including the tread and outer stabilizer ply.
As a result of the lack of definitive knowledge with respect to the cause or control of ply steer forces the industry approach has been generally to endeavor to optimize other force considerations or effect compensation for existing forces. Due to the significance attached to cornering stiffness forces, cords in the stabilizing plies of radial tires have generally been oppositely biased or oriented at a balanced angularity of approximately 68.degree. to a meridian line of the tire in the case of two or more stabilizer plies, since desirable cornering stiffness forces in a tire have been found to result thereform. There has been some suggestion in the prior art of the desirability of measuring lateral force characteristics of a tire after construction and forming suitable indicia on the tire such that it may be combined with other like tires also similarly marked such that a compensating or offsetting force balance may be effected during mounting on a vehicle. Such a compensation suggestion would be expensive in terms of the equipment and labor necessary for accurately effecting the force measurements, would constitute only an approximation since account is taken only of the direction and not the magnitude of the forces in different tires, and makes no pretense of eliminating the cause of such lateral forces. Understandably, there has been no adoption of this suggestion in any significant sector of the commerical market.